The objective of this project is to understand how enzymes tune their energy landscapes so as to perform their functions. It is well known that rather than existing as the static structures usually used to depict them, proteins are actually ensembles of conformational states. The fact that proteins undergo both structural and dynamic changes as part of the function indicates that the ensemble is critical to its function. The implication of this finding is that in addition to coding for the three dimensional structures, a protein's sequence also evolved to code for the entire energy landscape (i.e. the ensemble of conformations). It is of great import to know how this is done. Are there unifying principles that connect proteins with different functions? Here we leverage a unique discovery by our group during the previous grant period, which shows that the enzyme adenylate kinase (AK) from E. coli, uses local unfolding to modulate its enzymatic activity - in essence the energy landscape has unfolding within its functionally important repertoire. This mode of conformational change stands in stark contrast to the current accepted model, whereby an opening and closing reaction is believed to facilitate catalytic turnover. In the proposal, we leverage this unfolding reaction into a mutation strategy designed to investigate the coupling between the different regions of AK, and how that coupling produces an ensemble of states that constitutes the energy landscape of AK. Our approach is geared to understanding the role of conformational fluctuations in function, as well as reconciling the observations that, 1) dynamic changes in proteins can occur in the absence of structural changes; 2) function can be correlated to the stability of different regions of the protein; 3) binding can increase (rather than decrease) dynamics at many sites; and 4) structural and dynamic changes can propagate to distal parts of a protein structure in the absence of a pathway of structural or dynamic changes linking the two sites. We will perform binding and stability measurements using isothermal titration calorimetry (ITC), circular dichroism (CD) monitored thermal unfolding and hydrogen exchange (HX), and we will monitor the kinetics of the conformational and enzymatic processes using NMR 15N relaxation dispersion (CPMG) and steady state enzymatic analysis. Our studies are designed elucidate the states in the energy landscape of AK and to understand the role they play in driving the catalytic process.